Re: Possible improvement in DB structure

["brian m. carlson" <sandals@crustytoothpaste.net>]

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On 2019-12-23 at 13:00:46, Arnaud Bertrand wrote:
> Hello,
> 
> According to my understanding, git has only 3 kinds of objects:
> (excluding the packed version)
> - the blobs
> - the trees
> - the commits

There are also tags.

> Today to parse all objects of the same type, it is necessary to parse
> all the objects and test them one by one.

This isn't a behavior we often want.  Can you say more about why you
want to do this?

> May be due to my limited knowledge of git, I don't see any advantage
> to put everything together.
> By splitting the objects directory, the gain in performance could be
> important, the scripts simplified, the representation more clear.

Oftentimes, we want to look up an item that we would refer to as a
tree-ish.  That means that any tag, commit, or tree can be used in this
case and it will automatically be resolved to the appropriate tree.

Currently, we can look for any loose object, and then look for any
packed object, which is a limited number of lookups (at most, the number
of packs plus one).  Your proposal would have us look up at most the
number of packs plus six.

In addition, we sometimes know that we need to look up an object, but
don't know its type.  We would incur additional costs in this case as
well.

I'm not sure that we would gain a lot other than conceptual tidiness,
but we would incur additional performance costs.  We can currently
distinguish between the type of all of these objects by simply reading
the object header, which on a 64-bit system cannot exceed 28 bytes,
which we do in some cases, such as `git cat-file --batch`.
-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204

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